2003 Seattle Annual Meeting (November 2–5, 2003)
Paper No. 62-7
Presentation Time: 9:30 AM-9:45 AM

GEOCHEMISTRY AND PETROLOGY OF A MIOCENE TRACHYTE-BASANITE SUITE FROM MT. TSARATANANA, NORTHERN MADAGASCAR

BUCHWALDT, Robert, Department of Earth and Planetary Sciences, Washington Univ, St. Louis, Campus Box 1169, One Brookings Drive, St. Louis, MO 63130, buchwaldt@levee.wustl.edu, TUCKER, Robert, Earth & Planetary Sciences, Washington Univ, 1 Brookings Drive, Campus Box 1169, St. Louis, MO 63130-4899, DYMEK, Robert F., Department of Earth and Planetary Sciences, Washington Univ, Saint Louis, MO 63130, and CRISS, Robert E., Earth and Planetary Science, Washington Univ, One Brookings Dr, Saint Louis, MO 63130-4899

Mt. Tsaratanana, the highest peak in north-central Madagascar (~2876 m), is one of several Cenozoic eruptive centers that extend from the Seychelles south to the Comores Islands. Miocene volcanism (~10 Ma) is of peraluminous type and consists of less abundant basanitic flows and more abundant phonolitic and trachytic tuffs and flows. Phenocrysts of olivine (Fo88-90) and brown Al-, Ti-rich, subcalcic magnesian augite (XMg~0.9) occur in the basanites, whereas phenocrysts of Ti-magnetite, green ferroaugite (XMg~0.4), K-feldspar, and plagioclase (An70-80) occur within the phonolitic and trachytic suite. For the entire suite, SiO2 ranges from 44 to 66 with a large “Daly Gap” between 47-56 wt %, which is reminiscent of features found in volcanic rocks on many oceanic islands worldwide. In the basanites, as SiO2 increases, Cr, Ni, MgO and TiO2 contents decrease whereas Al2O3 and incompatible elements increase, indicating fractionation of mainly clinopyroxene, feldspar, amphibole, and Ti-magnetite. In the phonolites and trachytes, however, concentrations of Al2O3 and incompatible elements decrease but CaO/TiO2 increases with increasing SiO2, which we interpret as the result of increasing degrees of melting. Primitive mantle normalized trace element patterns are typical of ocean island basalts, showing characteristic hump patterns with high relative concentrations of Nb and Ta and pronounced negative anomalies of Ba, Sr, and Ti.  These patterns are are similar to those from the type localities of HIMU basalts, and also resemble patterns for basalts from the Comore hot spot trail.

Basanites, trachytes and phonolites have a restricted range in 87Sr/86Sr, 143Nd/144Nd (0.701-0.703 and 0.5127-0.5129, respectively), and d18O values are between 5.2 and 6.9 ‰.  Two basanites samples have average Pb isotope ratios of 206Pb/204Pb=19.53, 207Pb/204Pb=15.66 and 208Pb/204Pb=39.14, indicating a significant HIMU mantle component, consistent with trace element data.

The observed variations suggest that the Mt. Tsaratanana magmas may have originated from a sub-lithospheric HIMU reservoir similarly to other basaltic magmas generated throughout the Comores hot spot trace. The Sr- and O-isotope variations observed in trachytes and phonolites point to contamination of the magmas by only small amount of crustal materials (<5%).

2003 Seattle Annual Meeting (November 2–5, 2003)
Session No. 62
Volcanology and Igneous Petrology
Washington State Convention and Trade Center: 3A
8:00 AM-12:00 PM, Monday, November 3, 2003

Geological Society of America Abstracts with Programs, Vol. 35, No. 6, September 2003, p. 182

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